Chickens will be bred for endogenous avian leukosis viruses (ALVs). Cells with defined proviral contents will be used to study the RNAs and proteins expressed by different viruses. Cells with defined proviral contents will also be used to study phenotypic mixing and recombination of endogenous and exogenous ALVs. Chickens which express endogenous subgroup E envelope antigens and receptors for subgroup E virus will be studied for susceptibility to subgroup E ALVs and subgroup E ALV associated disease. An allele of 15B chicken which is associated with good growth of endogenous viruses will be studied to determine whether or not it codes for subgroup E virus receptors. Endogenous and exogenous viruses in commercial flocks will be characterized to determine their relatedness to known endogenous viruses and laboratory strains of exogenous viruses.